Synthesis, Structures, and Characterization of Transition Metal Ketimide and Hydrazonide Complexes

Transition metal ketimide (R2C=N-) complexes were synthesized. The mononuclear M(N=C-t-Bu2)4 (M = Zr, Hf, Cr, Mo, W, and Re) complexes were found to have tetrahedral (Zr and Hf), flattened tetrahedral (Cr, Mo, and W), and near square planar (Re) MN4 cores. An MO analysis suggests that the ketimide ligand functions as both a π donor and acceptor in the dn n > 0 complexes. The ligand π donor and acceptor properties are proposed to account for the observed spin pairing in Cr(N=C-t-Bu2)4, the first example of a diamagnetic Cr(IV) four-coordinate complex. The Cr and Mo complexes sublime cleanly at low temperatures under vacuum, suggesting they may be used as precursors for the chemical vapor deposition of thin films. The trinuclear ketimide complexes Co3(μ-N=C-t-Bu2)4(N=C-t-Bu2)2 and Fe3[μ- N=C(t-Bu)(Ph)]4[N=C(t-Bu)(Ph)]2 were found to have distorted trigonal-planar terminal metal centers connected to a central distorted tetrahedral metal center via bridging ketimide ligands. Solid-state magnetic studies on the Co complex and the previously reported Mn analog, Mn3(μ-N=C-t-Bu2)4(N=C-t-Bu2)2, were consistent with the trimers having antiferromagnetically coupled S = 3/2 and 5/2 metal centers, respectively. In both cases, the data were also consistent with weak intermolecular antiferromagnetic interactions at low temperatures. In solution, Fe3[μ-N=C(t-Bu)(Ph)]4[N=C(t-Bu)(Ph)]2 was found to have μeff = 5.5 μB by the Evans NMR method. The magnetic studies suggest that the ketimide bridging ligands facilitate intramolecular antiferromagnetic coupling between the metals in these complexes. The dinuclear ketimide complexes [Cr(μ-N=C-t-Bu2)(N=C-t-Bu2)]2, [Y(μ-N=C-t- Bu2)(N=C-t-Bu2)2]2, and {Y[μ-N=C(t-Bu)(Ph)][N=C(t-Bu)(Ph)]2}2 and tetranuclear ketimide complexes [Cu(μ-N=CR2)]4 (R = t-Bu, Ph) were synthesized and structurally characterized. The Cr(II) complex is the first diamagnetic dichromium complex with two bridging and two terminal non-chelating ligands. It has the smallest hinge angle in the Cr2(μ-N)2 core (91°) and the shortest Cr–Cr bond length (2.08 Å) among structurally related complexes. A σ2π4δ2 quadruple bond is proposed to account for the observed small hinge angle, short Cr–Cr bond length, and diamagnetism. The complex [Cu(μ- N=C-t-Bu2)]4 has short Cu⋅⋅⋅Cu contacts compared to copper compounds with similar structures. The novel hydrazonide square-planar complex Co[CH2C(t-Bu)=NNMe2]2 was synthesized. Based on a qualitative MO analysis, it is proposed to have a (dxy)2(dxz, dyz)4(dz2)1 configuration. A new hydrazonide ligand precursor, [LiCH2C(SiMe3)=N- NMe2]6, was synthesized and structurally characterized.